The headlong spread of mobile communication has in recent years led to the development of a number of improved radio transmission methods. To integrate the advantages of the Internet into the world of mobile communication, a Session Initiation Protocol (SIP), as it is called, was proposed, which is what is known as an “Application Layer Control Protocol”, i.e. a signaling protocol for establishing, modifying and terminating multimedia sessions. These multimedia sessions, can, for example, include multimedia conferences, Internet-telephone links (Voice over IP) and similar applications. By means of SIP, users can be invited, or added to, existing sessions and the composition of the multimedia session can be changed. The main capabilities of SIP are address allocation or address mapping, the localization of the called user and the diversion of connections. An SIP session in this case is usually realized between two communication terminals, also called user entities UE, with the interposition of several interconnected network elements, (e.g., proxies) being realized.
SIP was chosen by 3GPP as a signaling protocol for the IP Multimedia Call Network Subsystem (IMS). According to the current 3GPP specification, an SIP session can also be established or terminated by network elements (proxies), such as the P-CSCF, S-CSCF, MGCF or AS, within the IP multimedia core network subsystem [1], [2], [3] referenced at the end of the present text. The cause of this session termination by the P-CSCF can, for example, be the interruption of the radio link of the UE to the UTRAN (out of coverage). The S-CSCF ends the SIP session between the UEs because of a depleted pre-paid account of one of the terminals or for administrative reasons (see [1]).
However, the problem with this is that the SIP protocol specified in the Internet Engineering Task Force (IETF), and used by 3GPP in the IMS and adapted to this [purpose, the use of end-to-endprotocols. The control of the session is, by definition, in the logic units between which they are established. These logic units are known as user agents (UA), with these being distinguished as user agent clients (UAC) or user agent servers (UAS) depending on whether they transmit an SIP request or respond to a corresponding request with an SIP response (see [4]). Both logic units, UAC and UAS, are contained in a UE. Hence in accordance with the SIP protocol standard (IETF version), only the UAs can end an SIP session. This impedes the integration of the SIP protocol in mobile radio systems because this integration, as explained above, requires not only AUS, i.e. not only communication terminals, but also proxies as network elements, to be able to trigger a communication link.
FIG. 1 shows the known SIP signaling messages in the IMS during a termination of the SIP session initiated by P-CSCF, in a case where the radio link between the UE and UA and the UTRAN was broken (out of coverage) (see [1], FIG. 5.26a). As can be seen in this illustration, the session is established by the P-CSCF by means of the message designated by “hangup”. Hangup in this case stands for the transmission of the BYE request as it is known. The BYE message serves to identify the session to the P-CSCF and to indicate to the UE/UA that this session is to be terminated. Consequently, the UE or UA confirms, by means of the 200 OK message, the PSCF, the termination of the SIP session and then releases the PDP context, i.e. the data link to the UTRAN/CN on which the data of the media session is being transmitted. Further details are given in [1] chapter 5.10.3.1.1 (see also [2] chapter 8).
According to the prior art, a network element, e.g. the P-CSCF, thus terminates the SIP session. Accordingly, the UA has no control over who terminates the session or what proxy is entitled to do so. The UA functions purely as an “instruction receiver”. This deviating functionality of the standard SIP (3GPP version) adapted from 3GPP was deprecated by the IETF. According to the present state of the art, an integration of the SIP protocol into the 3GPP standard is not possible, at least not with the agreement of the IETF.